Recording materials using electron donating colorless dyes and electron accepting compounds are well known. For example, the recording materials are fully described in U.K. Patent 2,140,440, U.S. Pat. Nos. 4,480,052 and 4,436,920 and JP-A-62-144989 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Recording materials have been extensively studied to improve their characteristics such as (1) color density and color sensitivity, and (2) fastness of colored images in recent years.
The present inventors have paid attention to the characteristics of the electron donating colorless dyes and the electron accepting compounds such as solubility in oil and water, partition coefficient, pKa, the polarity of substituents, the positions of substituents, the crystallizability of mixtures and the change of solubility, and have investigated to develop good materials for recording materials and good recording materials.